Various plastics have been mass-produced from an oil-based resource (or a petroleum) as raw materials and widely used. Such plastics include, for example, a polyvinyl chloride, a polyolefin (e.g., a polyethylene, and a polypropylene), a polystyrene, a polyester (e.g., a polyethylene terephthalate, and a polybutylene terephthalate), and a polyamide such as a polyamide 6.
The above-mentioned plastics are generally durable and degrade very slowly in a natural environment. Most of the spent products comprising the plastics are incinerated, and unfortunately, only the limited kinds of the spent products are collected for recycling. The incineration of the spent plastic products generates carbon dioxide, which is a cause of global warming. Furthermore, the incineration of a plastic containing a halogen (such as a polyvinyl chloride) or a plastic containing a heteroatom (such as nitrogen, phosphorus, or sulfur) often tends to generate a toxic gas.
In addition, the petroleum is a limited resource. Since the plastic is made from the petroleum as a raw material, the decrease in the amount supplied of the petroleum or the depletion thereof will often cause an increase in the production cost of the plastic. In the worst case, the production is expected to be abandoned.
To overcome the above-mentioned problems, various researches, developments, and productions of biodegradable polymers, which are degraded by a bacteria or polymers which are degraded by hydrolysis or the like due to other factors under a natural environment, have been flourished in these years. In addition, the researches and developments of polymers which can be produced not from the petroleum, which is a limited resource, but from a renewable recourse as a raw material, have been progressing for recent years.
A polylactic acid, which is one of aliphatic polyesters, is biodegradable and can be produced not from the petroleum, but from a plant, which is a renewable recourse, as a raw material. For that reason, the polylactic acid has been attracting attention in recent years. The polylactic acid is usually produced by ring opening polymerization of lactide, which is a cyclic compound obtainable by dehydrating condensation of two lactic acid molecules by direct polymerization of lactic acid. Lactic acid, which is a raw material for the polylactic acid, can be produced by lactic acid fermentation of a saccharide as a raw material derived from a renewable plant [e.g., a saccharide obtainable by decomposition of a carbohydrate (such as starch) contained in grain (or corn) pulse crops, tubers (or corms) and a saccharide contained in a sugarcane].
The spent polylactic acid is left in a natural environment or buried in soil and gradually decomposed. The spent polylactic acid is also incinerated, which generates carbon dioxide as well as the incineration of the plastics produced from the petroleum as a raw material. However, the plant, which is a raw material for the polylactic acid, uses (or absorbs) carbon dioxide in the air at photosynthesis, in which the carbon dioxide is converted to a carbohydrate in the plant for the growth. Such a circulation of carbon dioxide through the generation by the incineration and the absorption by the plant forms a generation-consumption cycle of carbon dioxide. Owing to such a cycle the incineration of the polylactic acid dose not change the whole quantity of carbon dioxide in the air is the same after all, whereby the polylactic acid is an environment-friendly material. In this regard, the polylactic acid is more advantageous than the plastic produced from the petroleum as a raw material, since the plastic only generates carbon dioxide by the incineration.
Since the polylactic acid is produced at a relatively low production cost and has a transparency and a melt-moldability, the polylactic acid is used for a film, a sheet, or the like. However, since the polylactic acid is hard (or rigid) and breakable (or fragile) and has a poor flexibility or impact resistance, the polylactic acid is not suitable to use. Such disadvantages require to be overcome. To improve the shortcomings, for example, a plasticizer is added to the polylactic acid to imparting flexibility thereto. However, the bleed out of the plasticizer tends to occur, which spoils the effect of the plasticizer.
Accordingly, Japanese Patent Application Laid-Open Publication No. 286401/2003 (JP-2003-286401A, Patent Document 1) suggests a polylactic acid composition comprising a polylactic acid and an unsaturated carboxylic alkyl ester-series polymer having a weight-average molecular weight of not more than 30,000 to impart a flexibility thereto without deteriorating the transparency of the polylactic acid. In the polylactic acid composition of the Patent Document 1, practically, a homopolymer of an alkyl acrylate, a copolymer of alkyl acrylates, a random copolymer of an alkyl acrylate and a methacrylate, which independently has a weight-average molecular weight of not more than 5000, is used as an unsaturated carboxylic alkyl ester-series polymer to improve the flexibility. While the flexibility is enough improved, the composition still has a poor impact resistance and mechanical properties, a sticky texture, or the like. In some cases, the bleed out of the unsaturated carboxylic alkyl ester-series polymer occurs.
Moreover, Japanese Patent Application Laid-Open Publication No. 269720/2004 (JP-2004-269720A, Patent Document 2) suggests a polylactic acid composition comprising a polylactic acid and an acrylic polymer having a methyl methacrylate unit and an alkyl acrylate unit to improve the heat resistance, with maintaining the transparency of the polylactic acid. In the invention of Patent Document 2, it is recommended that an acrylic copolymer containing a methyl methacrylate unit of not more than 50% by weight and an alkyl acrylate unit of not less than 50% by weight be used as the acrylic polymer to improve the heat resistance. According to the invention of Patent Document 2, while the heat resistance is improved in some degree, unfortunately, the obtainable polylactic acid composition and a molded product thereof do not have excellent flexibility, impact resistance, flex fatigue resistance, and thermal adhesiveness. In addition, the composition and the molded product are not oil resources-saving.
Furthermore, Japanese Patent Application Laid-Open Publication No. 155207/2002 (JP-2002-155207A, Patent Document 3) suggests a thermoplastic polymer composition comprising a polyester resin (such as a polyethylene terephthalate or a polylactic acid) and an acrylic polymer having a number-average molecular weight of 800 to 20000, in order to obtain a thermoplastic polymer composition having an excellent weather resistance and moldability. In Patent Document 3, an acrylic acid-α-methylstyrene-styrene copolymer, a 2-ethylhexyl acrylate polymer, or the like is used as the acrylic polymer. Unfortunately, the thermoplastic composition of Patent Document 3 still has a poor flexibility, impact resistance, flex fatigue resistance, transparency, heat resistance, or the like.
Moreover, WO 2002/092696 publication (Patent Document 4) suggests a thermoplastic resin composition comprising (a) a thermoplastic resin and (b) a block copolymer comprising (A) a methacrylic polymer block and (B) an acrylic polymer block. In Patent Document 4, a polylactic acid is an example of many thermoplastic resins included in the thermoplastic resin (a). In Examples, a polybutylene terephthalate, a polyamide, a polyvinyl chloride, and a methacrylic resin are used. The document discloses an A-B-A type triblock polymer as the block copolymer (b). The A-B-A type triblock polymer is obtainable by living polymerization using a halogen-containing compound as an initiator. However, the use of the triblock polymer of Patent Document 4 deteriorates the heat resistance of the composition, and tends to cause the bleed out of the components from the composition or a molded product thereof. In addition, coping with both flexibility and toughness is difficult.    [Patent Document 1] JP-2003-286401A    [Patent Document 2] JP-2004-269720A    [Patent Document 3] JP-2002-155207A    [Patent Document 4] WO2002/092696A